The invention relates to an implantable sound receptor for hearing aids, in particular for implantable hearing aids.
The majority of the known hearing aids is unsuitable for implantation. In principle, such hearing aids employ converters that are able to convert sound waves into electric signals. Such converters are known in the form of microphones and require appropriate membranes whose vibrations can be transformed into electric signals. The sensitivity of such microphones is largely influenced by the site of attachment and, above all, the size of the membrane. Acoustic vibrations can be picked up by pressure-sensitive membranes as is usually the case with microphone structures, or they can be detected by vibrometers receiving vibrations as acceleration signals or even as expansion measuring signals at deformations of vibrating structural components.
In U.S. Pat. No. 5,531,787, accelerometers in the form of piezo-resistive vibration sensors have been proposed. Alternatively, capacitive acceleration sensors are known to scan sound vibrations. Such miniaturized sensors have already been proposed for implantation in the region of the middle ear, whereby acoustic pressure waves generated in the region of the middle ear are scanned in the form of mechanical vibrations. In principle, such microphone structures are, however, relatively insensitive, because precise tuning to the acoustic impedance between the sensor and the tympanic cavity of the middle ear is not readily feasible.
Also other references such as, for instance, U.S. Pat. No. 3,557,775 have described microphones which may be implanted under the skin to receive audio signals, transmission being effected into the middle ear. Such arrangements, too, in terms of sensitivity are subjected to a number of not readily controllable foreign influences such as, for instance, the thickness of the skin and an unforeseeable formation of scar tissue and granulation tissue during the healing process, so that the sensitivity at the frequencies that are important to hearing will be attenuated in a variable and uncontrolled manner.
In principle, sounds that are heard are generated by sound waves, the pitch of a tone increasing with increasing frequency and the loudness increasing with increasing amplitude. In addition to tones and sounds, which constitute tonal mixtures, also a plurality of tones irregularly sounding simultaneously and having different frequencies and pitches are generated, which are heard as noises. By the natural hearing process, those sound waves are distinguished which are conducted from the auricle to the external auditory, thus causing the tympanic membrane to vibrate. The tympanic membrane is grown together with the hammer shank, whereby further transmission is effected via the auditory ossicles through the base of the stapes to the perilymphatic liquid, which causes the organ of Corti to vibrate. The excitation of the hair cells in the organ of Corti cause the generation of nerve impulses which are conducted by the auditory nerve into the brain, where they will be distinguished consciously.
The tympanic membrane functions as a pressure receiver and has a diameter of about 1 cm. If microphones having such large membranes are to be used to receive sound waves, such microphones will hardly be suitable for implantation, since the space required therefor is not available in the region of the ear.
Impaired hearing may have various causes. In a considerable number of hearing defects, the mechanical part of the vibration transmission from the tympanic membrane via the auditory ossicles to the liquid in the scala vestibuli is intact. It has, therefore, been already proposed to connect a vibration sensor directly with the membrane or the auditory ossicles in order to accordingly convert into electric signals, and amplify, the vibrations brought about by sound. Such an intervention has the drawback that, on the one hand, relatively high operating expenditures are involved in the arrangement of such sensors and, on the other hand, every mechanical influence of vibrating parts and, in particular, the damping of such vibrating parts, considerably influences the vibration behavior of those parts such that correct signals as are generated during the natural hearing process will not be obtained in that case, either. In principle, the miniaturization of microphones results in a decrease of sensitivity, this being not least due to the missing tuning of the acoustic impedance between microphone and ambient air. Even if this effect can be improved by implanting the microphone under the skin, this will lead to an alteration of the scannable frequency range with, in particular, higher frequencies being more strongly attenuated. Also other mechanical sound wave receptors such as, for instance, tubes filled with fluids, lead to an attenuation due to the viscosity of the used fluid, whereby rigid acoustic couplers, in general, are unsuitable for implantation.
The invention aims to provide a small-structured implantable sound receptor which avoids the drawbacks of the known sound receptors while maintaining the acoustic sensitivity on a constantly high level over the entire frequency range that is essential to hearing, i.e., from about 100 Hz to more than 10 kHz. The invention, furthermore, aims to keep the structural dimensions so small as to enable its implantation in the middle ear and/or the adjacent mastoid antrum. The surgical intervention preferably is to be reversible, whereby no substantial deterioration of the previously existing audition is to occur at a functional failure of the sound receptor. In a restrictive manner, an operative interruption of the sound transmission chain may, however, be required as a function of the employed actuator and its point of application, in order to avoid feedbacks. In addition to these demands set on an implantable sound receptor, also the energy consumption of the sound receptor and of a consecutive evaluation circuit is, of course, to be kept so low as to enable a total implantation due to miniaturization.
To solve this object, the implantable sound receptor for implantable hearing aids according to the invention essentially consists in that the sound sensor is designed as an optical sensor for vibration and distance measurements and is arranged within the ear at a distance from the surface of a sound transmission part capable of being excited to acoustic vibrations. Due to the fact that, as opposed to the hitherto applied physical principles of sound receptors for hearing aids, contactless scanning by means of an optical scanner has been proposed, it is actually feasible to measure the vibrations that are physiologically transmitted by the tympanic membrane and the auditory ossicles. The contactless configuration prevents undesired side effects of an attenuation of such vibrating auditory ossicles and the tympanic membrane and enables the unhampered use of the relatively large vibration pickup surface of the tympanic membrane for measuring, so that a considerably higher sensitivity can actually be attained than would be feasible with accordingly smaller membranes. Due to the fact that the optical sensor is arranged or arrangeable within the ear at a distance from the surface of a vibration transmission part capable of being excited to vibrations, it is ensured that any attenuation of the vibrations of such vibration transmission parts capable of being excited to vibrations will be reliably excluded and the use of optical sensors will allow the use of extremely small-structured sensors.
By optical sensors, sensors that do not necessarily use visible light are to be understood in this context. For optical sensors, electromagnetic waves in a relatively wide frequency range that goes beyond the spectrum of visible light may be used. As transmitters, laser diodes may, in particular, be used both in the infrared and ultraviolet ranges of radiation and in the visible range, as long as the vibrating surface to be measured is sufficiently reflective in the range of the irradiated wavelength. Optical sensors, thus, in the first place serve to measure the optical parameters of the reflected portions of the emitted signal, the evaluation of the signals of the sound receptor advantageously being effected in a manner that the optical sensor is connected with an interferometer to evaluate the amplitude, the frequency and/or the relative phase relationship of the vibration of the scanned part. The use of the interferometer principle, for which various configurations are known, allows the reliable detection of even slight amplitudes of natural vibrations in the region of the auditary ossicles in a contactless manner. The range to be detected extends from amplitudes of 10xe2x88x9211 m to approximately 10xe2x88x925 m, whereby higher amplitudes than approximately 5xc3x9710xe2x88x925 m as may be observed at a sound irradiation of about 120 dB, as a rule, will not be taken into consideration for further measurements since these may already cause damage to the internal ear.
The vibrations of the auditory ossicles and the tympanic membrane as observed upon excitation by acoustic waves, however, are overlaid in the ear also by a low-frequency quasi-static or slow dislocation of the tympanic membrane and the ossicles, which is due to differences in the air pressure or the pressure prevailing within the internal ear. Such low-frequency dislocations are caused, for instance, by changes in the air pressure when going by a lift, cable car or airplane, whereby important low-frequency deviations are also observed when blowing one""s nose due to the sudden opening of the auditory tube. Such low-frequency dislocations in terms of amplitude may be higher by a factor of at least 102 than the maximum amplitudes occurring at the physiological exposure to sonic waves. Thus, optical sensors must be arranged in a manner that no contact with the part to be scanned will occur even during such dislocations, and the configuration according to the invention is, therefore, devised such that the optical sensor is arranged at a distance from the scanned part that is larger than the maximum dislocation of the scanned part occurring in the direction of the sensor, and/or is adjustably retained at a collision-preventing distance. The use of an adjustable retention means to maintain a defined distance may comprise a servomotor, the adjustment signals of the servo-motor being applied to determining the acoustic vibrations and the adjustment movements themselves being triggered by the optical sensor.
Optical scanning in a particularly simple manner is feasible in that the optical sensor cooperates with at least one light or laser diode and the reflected signals are fed to at least one optoelectronic switching component, for instance a photodiode, of an electronic evaluation circuit via fibers of waveguides, in particular optical waveguides. The sensor part to be implanted in the middle ear or in the epitympanum or epitympanic recess in such a configuration is restricted to the relatively small free end of the optical waveguide via which the optical signals are fed and the reflected signals are picked up. In the beam path also one or several optical systems such as, for instance, lenses, beam splitters, prisms, mirrors or the like may, of course, be arranged depending on the orientation and structure of the device, in order to render measurements accordingly precise or locate the same.
The evaluation circuit subsequently must provide an accordingly amplified signal to the stimulus of the auditory nerve, the configuration in this respect advantageously being devised such that the evaluation circuit generates signals for the electromechanical vibration generator, and/or the electric stimulation of the organ of Corti and/or the auditory nerve and/or the brain stem, and comprises connections for the respective signal lines.
In order to prevent the free end of the optical sensor to be implanted preferably in the middle ear from being subjected to measuring errors or sensitivity fluctuations caused by turbidities, the configuration advantageously is devised such that the free ends of the optical sensor are provided with a cell-growth-inhibiting coating.
In order to suppress fading effects which are to be observed in the interferometric evaluation and overlay of low-frequency dislocations, it is particularly advantageous to detect also the relative phase position of the vibration of the scanned part in addition to evaluating its amplitude and frequency. To this end, it may advantageously be proceeded in a manner that at least two signals are fed to the evaluation circuit to determine the phase relationship, the determination of the phase relationship in a known manner enabling the respective active or passive stabilization as a function of the type of interferometer used and the circuit arrangement of the evaluation circuit chosen. Depending on the arrangement, the optimum sensitivity of the optical sensor is based on a predetermined operating point pregiven by a defined distance relative to the surface to be measured. Low-frequency dislocations of the parts to be scanned naturally may lead to the leaving of that optimum operating point or even to the occurrence of a phase shift or phase reversal. Such undesired side effects, which may cause xe2x80x9cfadingxe2x80x9d of the measured signal advantageously may be eliminated in that the evaluation circuit comprises a stabilizing circuit to compensate for the shift of the operating point of the interferometer by low-frequency dislocations of the scanned part. Alternatively or additionally, appropriate compensation may be safeguarded in that a sensor for the detection of the distance of the part to be scanned from the optical sensor is additionally provided. The stabilization of interferometric signals may be effected in a particularly simple manner by comparison with a reference signal or by measuring a plurality of signals, whereby polarizing beam splitters may be arranged in the beam path and the signals may be detected independently and by different photodiodes. Conclusions as to the correct phase position may be drawn also from a mathematical analysis of the shape of the measuring signal, frequency comparisons and, in particular, the evaluation of higher-order vibrations in the stabilizing circuit being applicable to this end.
In order to enable the precise positioning of the sound receptor, the configuration in a particularly simple manner is devised such that the free end of the optical sensor is adjustably fixed in a bearing block and/or connected with an adjustment drive, thus safeguarding the precise orientation and precise positioning relative to the surface of the part whose vibration is to be measured.
The actual configuration of the interferometer subsequently calls for the respectively preferred algorithms for evaluation. Interferometers in this regard may be configured according to any desired mode of construction such as, for instance, as a Michelson, Fabry-Perot or Fizeau interferometer, suitable stabilizing algorithms being described, for instance, in the article by K. P. Koo, A. B. Tveten, A. Dandridge, xe2x80x9cPassive stabilization scheme for fiber interferometers using (3xc3x973) fiber directional couplersxe2x80x9d, in Appl.Phys.Lett., Vol. 41, No. 7, pp. 616-18, 1982; G. Schmitt, W. Wenzel, K. Dolde, xe2x80x9cIntegrated optical 3xc3x973 coupler on LiNbO3: Comparison between theory and experimentxe2x80x9d, Proc.SPIE, Vol. 1141, 5th European Conference on Integrated Optics: ECIO 89, pp. 67-71, 1989, R. Fuest, N. Fabricius, U. Hollenbach, B. Wolf, xe2x80x9cInterferometric displacement sensor realized with a planar 3xc3x973 directional coupler in glassxe2x80x9d, Proc.SPIE, Vol. 1794, Integrated Optical Circuits II, pp. 352-365, 1992; L. Changchun, L. Fei, xe2x80x9cPassive Interferometric Optical Fiber Sensor Using 3xc3x973 Directional Couplerxe2x80x9d, Proc.SPIE, Vol. 2895, pp. 565-571, 1995. Other proposals are to be found, i.a., in A. Dandridge, A. B. Tveten, T. G. Giallorenzi, xe2x80x9cHomodyne Demodulation Scheme for Fiber Optic Sensors Using Phase Generated Carrierxe2x80x9d, IEEE J.Quantum Elec., Vol. QE-18, No. 10, pp. 1647-1653, 1982, J. H. Cole, B. A. Danver and J. A. Bucaro, xe2x80x9cSynthetic Heterodyne Interferometric Demodulationxe2x80x9d, IEEE J.Quantum Elec., Vol. QE-18, No. 4, pp. 694-697, 1982.